Measurement of Microscopic Viscosity of Fluorescent Molecular Probe Doped in Biological Cell

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Malachite green dye molecules (MG), a kind of triphenyl methane dye, are known for their very rapid nonradiative relaxation from the optically excited state S_1 to the ground state S_0. In MG the central carbon is joined by three phenyl rings and the excited state dynamics is strongly influenced by the microscopic viscosity of the local environment. The viscosity-dependent nonradiative process is thought to be due to diffusive rotational motion of the phenyl rings of MG around their axes, which causes the internal conversion from the excited states to the ground state. The fluorescence decay time is essentially determined by the microscopic viscosity of the local environment.The application of the fluorescent microprobe to biological objects is quite interesting, because the fluorescence measurement is very sensitive and noninvasive. We measured the fluorescence decay time of MG doped in onion cells by identifying one cell to another or even the position in a cell by using an optical microscope. It was found out that the decay time varies depending on the position in a cell, or one cell to another. Although the origin of the difference of the fluorescence decay time is not known at present, the microscopic viscosity of each site is different and it may reflect the difference of the biological activity of each cell. In recent years photon counting is possible to measure the very weak fluorescence. If this method can be combined with optical fibers, it may serve as a medical diagnostic method in the future.